This invention relates to a toner for developing an electrostatic latent image in electrophotography, electrostatic recording, electrostatic printing and so on, and to a method for producing the toner. More particularly, the present invention is directed to a black toner without using carbon black.
Carbon black has been conventionally used as a black colorant for a toner for developing an electrostatic latent image in image forming machines such as copying machines, laser printers and facsimile machines. However, because of a problem of carcinogenesis, the use of toners without carbon black has been long desired. While a magnetic material is a black colorant, the use thereof is limited to a magnetic toner. An organic black colorant such as aniline black and a Nigrosine dye has a problem because a high density image is not obtainable.
Japanese Patent No. 2736680 proposes a black metal oxide pigment composed of Fe2TiO5 and Fe2O3—FeTiO3 and a particle diameter of 0.1 to 0.5 μm. While this black pigment is safe, non-magnetic and heat resistant, it is necessary to use a large amount, i.e. 40 to 60% by weight based on the weight of the toner, in order to obtain color density comparable to the known carbon black toner. Because of high specific gravity of the black pigment, however, the toner containing such a large amount of the black pigment has 1.4 to 1.8 times as great a specific gravity as that of the conventional carbon black toner. Thus, the black metal oxide pigment poses a lot of problems such as occurrence of abnormity in a toner density sensor or a toner amount sensor and short lifetime of carriers.
Japanese Patent No. 2997206 proposes a toner containing a black metal oxide pigment composed of oxides of cobalt, manganese and iron and having a specific surface area of 50 to 100 m2/g. Because of the extremely fine particle size, this black pigment can afford image density comparable to the conventional carbon black colorant even when used in an amount of 10 to 30% based on the weight of the toner. However, the pigment is apt to form an aggregate and is not uniformly dispersed in the toner. As a consequence, the charging amount of the toner is insufficient and the developing efficiency is not good.